R (disambiguation)

Look up R or r in Wiktionary, the free dictionary.

R, or r, is the eighteenth letter of the English alphabet.

R or r may also refer to:

Science

Arginine, an amino acid abbreviated as Arg or R
ATC code R Respiratory system, a section of the Anatomical Therapeutic Chemical Classification System
Coefficient of relationship (r), in biology
Effective reproduction number (R), the number of cases generated by one case in the current state of a population in epidemiology
- Basic reproduction number (R₀), the expected number of cases directly generated by one case
Haplogroup R (mtDNA), a human mitochondrial DNA (mtDNA) haplogroup
Haplogroup R (Y-DNA), a Y-chromosomal DNA (Y-DNA) haplogroup
Net reproduction rate (R₀), the average number of offspring that would be born to a female given conforming conditions
r, the population growth rate in the r/K selection theory of ecology

R (cross section ratio), the ratio of hadronic to muonic cross sections
Electrical resistance (R)
Roentgen (unit) (R), a unit of measurement for ionizing radiation such as X-ray and gamma rays
Rydberg constant (R_∞, R_H), a physical constant relating to energy levels of electrons within atoms
- Rydberg unit of energy (R_y), the energy of the photon whose wavenumber is the Rydberg constant

R-value (insulation), a non-SI measure of thermal resistance used in housing insulation
R-value (soils), property of soils used in pavement design
Resistor, an electronic component
R-, a refrigerant numbering system
R, the ratio between the minimum and maximum stresses in fracture mechanics
R- or recall-button, now mostly used to switch between two calls

R (film), a 2010 film by Danish screenwriter and film director Tobias Lindholm
R, a character played by John Cleese in The World Is Not Enough
R, the protagonist of Warm Bodies, based on the book of the same name
R, a film rating in the Motion Picture Association of America film rating system and in the Canadian Home Video Rating System

Nissan Skyline GT-R, commonly referred to as the R in Japan
R (New York City Subway service)
Rinkai Line, a railway line operated by the Tokyo Waterfront Area Rapid Transit, labeled R
Transilien Line R, a line of the Paris transport network
Reconnaissance aircraft (military designation: R)
Restricted airspace
Volkswagen R, the performance division of Volkswagen
The official West Japan Railway Company service symbol for:
- Hanwa Line.
- San'yō Main Line.